Trailing edge vortex shedding in hydraulic turbines and the effect of stream-wise vorticity on vortex induced vibrations

Abstract

Experiments as well as some numerical simulations were conducted investigating the resonance region of vortex induced vibrations for different trailing edges of a hydrofoil in channel flow. The study focuses on the usefulness of stream-wise vorticity for the mitigation of vortex induced vibrations for the different trailing edge geometries. Though applicable to several different engineering disciplines, the topic is pursued in the context of hydraulic machinery. To this end, flow measurements were also conducted in the vane-less space of a high head model Francis turbine, to investigate the role trailing edge modifications of guide-vanes might play in such machines. The flows were studied by use of particle image velocimetry. In the case of the individual hydrofoil vibrations were measured by means of embedded strain gauges for different trailing edge geometries. Purely computational fluid dynamics simulations were carried out to investigate different Reynolds-averaged Navier-Stokes based turbulence models capability to predict vortex shedding frequencies and wake velocity profiles for one geometry considered to be representative for hydrofoils utilized in hydraulic turbines. Most notably, it was found that stream-wise vortexes, here introduced by the means of sub-boundary layer vortex generators, can substantially reduce the amplitudes of the vortex induced vibrations for a hydrofoil, even under resonant conditions. The set of measurements and initial numerical simulations lay the ground-work for further investigations and optimizations of such passive flow control devices.